The explosive growth in mobile electronic devices such as smartphones and tablets creates an increasing need in the art for compact and efficient switching power converters so that users may recharge these devices. A flyback switching power converter is typically provided with a mobile device as its transformer provides safe isolation from AC household current. A conventional flyback converter that uses a rectifying diode at the secondary (load) side of its transformer has significant power loss due to a relatively high forward voltage drop in the rectifying diode. Thus, synchronous rectification techniques have been developed that replace the rectifying diode with an actively controlled switch such as a field-effect transistor (FET) device (e.g., a metal oxide field-effect transistors (MOSFET) device) to improve operating efficiencies by taking advantage of its lower power losses.
Conventional flyback converters with synchronous rectification typically include a synchronous rectifier controller that controls the synchronous rectifier switch (S2) based on a voltage across the synchronous rectifier switch terminals. When this voltage falls below an on-time threshold voltage, the controller switches on the synchronous rectifier switch S2 so that power is delivered to load. During this power delivery, the voltage across the synchronous rectifier switch S2 gradually rises above the on-time threshold voltage until it crosses an off-time threshold voltage. This off-time threshold voltage corresponds to the voltage across the synchronous rectifier switch at the transformer reset time when the secondary winding current has ramped down to zero.
The timing of the on and off states for the synchronous rectifier switch is critical to reduce losses. But the control with regard to the on-time and off-time threshold voltages is problematic because the voltage across the synchronous rectifier switch S2 will have a resonant oscillation at the switch on times and off times due to parasitic effects. When the synchronous rectifier switch S2 is switched on, this resonant ringing could cause the switch voltage to exceed the off-time threshold voltage such that the controller would undesirably switch off the synchronous rectifier switch even though the secondary winding current is still relatively robust (it being well before the transformer reset time). To prevent such an undesirable premature cycling off of the synchronous rectifier switch S2, it is conventional for the controller to apply a minimum on-time period with regard to monitoring the off-time threshold voltage following the cycling on of the synchronous rectifier switch S2. During this minimum on-time period, the controller does not respond to the synchronous rectifier switch S2 voltage exceeding the off-time threshold voltage.
An analogous minimum off-time period follows the cycling off of the synchronous rectifier switch S2 to prevent the controller from responding to a resonant oscillation of the switch voltage that causes the switch voltage to fall below the on-time threshold voltage. But in contrast to the resonant oscillation that occurs at the synchronous rectifier switch S2 on-time, the resonant oscillation at the switch off-time is markedly more robust and prolonged. This robust off-time oscillation of the switch voltage complicates the setting of an appropriate duration for the minimum off-time period that may be better appreciated through a consideration of the waveforms shown in FIG. 1 for a power cycle 110 and a power cycle 120 of a primary-side power switch S1. In this example, the voltage across the synchronous rectifier switch S2 is a drain-to-source (VD-S) voltage for a MOSFET. In response to the cycling off of power switch S1, the drain-to-source voltage for synchronous rectifier switch S2 falls below the on-time threshold voltage. The synchronous rectifier switch S2 is thus switched on while at the same time a timer (S2 Min TON Timer) is started to time the minimum on-time period. The resulting resonant oscillation of the drain-to-source voltage is relatively minor and quickly damped such that the duration of the minimum on-time period may be relatively short.
In response to the cycling on of the synchronous rectifier switch S2, the secondary winding current pulses on and begins to ramp down until it reaches zero at the transformer reset time (T1 Reset). At the same time, the drain-to-source voltage for switch S2 rises above the off-time threshold voltage (S2 OFF Threshold) such that the S2 switch is switched off and a timer (S2 MIN TOFF Timer) begins timing the minimum off-time period. The resulting resonant oscillation for the drain-to-source voltage following the synchronous rectifier switch S2 off time is more pronounced and slower to damp as compared to the damping that occurs at the on time for synchronous rectifier switch S2. For power cycle 110, the minimum off-time period has a proper duration such that the resonant oscillations of the drain-to-source voltage do not cross the on-time threshold voltage following the termination of the minimum off-time period.
But the resonant oscillations following the synchronous rectifier switch S2 off time are more pronounced for a subsequent power cycle 120 of power switch S1. Due to this more pronounced resonance, the drain-to-source voltage crosses the on-time threshold voltage at a time 122 following the termination of the minimum off-time period in power cycle 120. As a result, the controller cycles the synchronous rectifier switch S2 on despite there being no power pulse to deliver. The result is that the secondary winding current has a slightly negative value during the minimum on-time period following time 122. Upon the termination of this undesirable minimum on-time period, the drain-to-source voltage exceeds the off-time threshold voltage such that the synchronous rectifier switch S2 is cycled off for another minimum off-time period. But the subsequent resonant oscillation of the drain-to-source voltage again causes the drain-to-source voltage to cross the on-time threshold voltage such the synchronous rectifier switch S2 is again cycled on a time 124. Another negative current is induced on the secondary winding until the termination of the subsequent minimum on-time period whereupon the drain-to-source voltage again exceeds the off-time threshold voltage such the synchronous rectifier switch S2 is opened.
The resulting cycling on and off of the synchronous rectifier switch S2 following the transformer reset time is undesirable for a number of reasons. For example, the negative current excited across the secondary winding wastes power. More fundamentally, the synchronous rectifier switch S2 may be cycled on when the power switch cycles on, which is a severe problem. The prior art setting of the minimum off-time period is thus problematic in that it cannot be set too short or this undesirable cycling of the synchronous rectifier switch S2 occurs yet it cannot be set too long in that the minimum off-time period would then interfere with the next power switch S1 cycling.
Accordingly, there is a need in the art for improved synchronous rectifier control techniques for switching power converters.